Electrorheological (ER) fluids have been known for a number of years in the form of colloidal dispersions of particles, usually polymeric, in a low conductivity medium, which fluids undergo a dramatic change in physical properties, e.g. viscosity when exposed to an electric field. ER fluids are capable of changes in physical properties within a millisecond and should promptly revert to their initial (zero field) condition upon removal of the electric field.
The ability of ER fluids to respond rapidly to electrical fields should theoretically allow ER fluids to take the place of certain electromechanical components. The application of an electric field increases the viscosity and yield stress of the fluid. In a sufficiently strong field, the ER fluid changes to a semi-solid, which supports shear stress allowing for transfer of energy, e.g. torque. ER fluids therefore should be particularly useful in providing a rapid response interface between electronic controls and mechanical devices, thereby increasing the speed and number of repetitions the device can perform.
ER fluids are proposed for use in electromechanical clutches, as described in U.S. Pat. No. 5,073,282 to Ahmed. U.S. Pat. No. 5,000,299 to Goto et al. describes a shock absorber including a cylinder chamber which is divided into two chambers containing ER fluid, and ER fluids are considered to have substantial applications as shock absorbers. U.S. Pat. No. 5,094,328 to Palmer describes a clutch system employing an electrorheological fluid which transmits torque between two sets of interleaved plates. U.S. Pat. No. 4,840,112 to Bhadra et al. describes an electrically controlled combined valve/cylinder apparatus containing ER fluids. Other promising applications include fluid-filled engine mounts, high speed valves with no moving parts, and interfaces between the electronic and mechanical parts of machines.
Electrorheological (ER) fluids are generally considered to consist of four components: the fluid medium or dispersing vehicle, the particles, a polar liquid or activator, and optional stabilizers which function to keep the particles dispersed in the absence of an electrical field. The fluid medium is a nonpolar, hydrophobic, electrically-insulating liquid having a low dielectric constant and generally a permittivity less than that of the particles, such as mineral oil, silicone oil, and aliphatic hydrocarbons, particularly chlorinated hydrocarbons. The particles are generally hydrophilic substances, such as silica-gel, starch, ionic polymers, e.g. alginic acid, polymethacrylates, phenolformaldehyde resins, and other synthetic polymers. Particle size is important. A size large enough to overcome kinetic forces imposed by the suspending medium is required; however, particles should be small enough to move freely and to have gravitational and buoyancy forces nearly equal. Particles having a size of about 30 .mu.m are typical, and the partial volume fraction occupied by particles in ER fluids is often near 25%. The activator is generally a polar liquid, such as water or an alcohol or other liquid which contains amine or alcohol groups, e.g. ethylene glycol, diethylamine, or the like. Such water may contain dissolved salts. The activator coats the surfaces of the particles allowing the particles to become polarized under an electric field. A stabilizer, such as a surfactant, is optionally added to maintain the particles dispersed in the ER fluid in the absence of an electrical field. Surfactants known in the art include fatty acid esters, fatty amines, glycerol, and glycerol esters.
The controllable behavior of ER fluids is believed to be caused by the induced polarization of the particles when an electric field is applied. The polarized particles then interact to form a filamentary network structure, which results in increased viscosity as illustrated in T. C. Halsey, Science, 258, 761-766 (1992).
This mechanism of operation at a molecular level is also described in a survey article entitled "Electrorheological Fluids" by T. C. Halsey and J. E. Martin which appears in the Oct. 1993 issue of Scientific American.
A number of problems with the use of ER fluids have prevented their widespread commercial application. One problem affecting the use of ER fluids is the tendency of the particles to clump and/or settle out of the fluid under gravitational force. Such settling of particles disturbs the ability of the particles to form an internal network upon polarization under an electric field. Attempts have been made to correct the tendency to sediment; for example, U.S. Pat. No. 5,032,307 to Carlson discloses the use of anionic surfactant compositions which are designed to act as both the particle component and as a surfactant to maintain a particle dispersion. U.S. Pat. No. 4,990,279 to Ahmed discloses the formation of hydrophilic shells or globules around hydrophobic polymers to maintain dispersion of particles. There remains a need however, for improved means for maintaining particle dispersions in ER fluids.
Another problem involved in the use of ER fluids is the difficulty in confining them, and seals are generally required to prevent leakage. An additional problem encountered in using ER fluids in mechanical devices is that some particles may have an abrasive effect on the surfaces of mechanical parts, requiring the shielding of these surfaces from contact therewith. Still another potential problem with ER fluids is the difficulty encountered in stably locating one or more electrodes in association with such a liquid medium.
Therefore, it is an object of the present invention to provide a material which overcomes the negative limitations of ER fluids as they are presently available. It is a further object of the present invention to provide means for preventing the settling of ER fluid particles, leakage of the ER fluid, abrasion of mechanical surfaces, and for facilitating the stable location of electrodes in association with ER fluids. Still another object is the introduction of the concept of a solid polymer structure of generally cohesive character into the field in contrast to traditional ER fluids used heretofore. Yet another object is to provide a "smart" solid material which can adapt its physical properties as desired in a closely controlled behavior or manner.